According to Freud,what Causes a Child to Enter a New

According to Freud,what causes a child to enter a new psychosexual stage of development?

A) There is a shift in which part of the body is the focus of pleasure.
B) The id has become a more powerful force in the child's life.
C) The id has become a less powerful force in the child's life.
D) The type of pleasure received from the genitals has changed.